The present invention relates to magnetic regenerator materials for cryocoolers comprising ductile intermetallic compounds, which order magnetically below 30 K, and, more particularly, to magnetic regenerators to enhance the cooling power and efficiency and closed cycle cryocoolers operating from approximately 300 K to approximately 2 K.
Regenerators are an integral part of cryocoolers to reach low temperatures between 4 K and 20 K (approximately 270 to 250 K below room temperature) regardless of the refrigeration technique employed; e.g., regardless of whether the known Gifford-McMahon, Stirling, pulse tube, etc. cooling technique is employed. A two stage Gifford-McMahon cycle cryocooler or refrigerator used to reach extremely low temperatures, such as approximately 10 K, without a liquid refrigerant is discussed in U.S. Pat. No. 5,186,765. For discussion of other cryocoolers, see books entitled “Cryogenic Heat Exchangers”, Plenum Press, New York, 1997, by R. A. Ackerman and entitled “Cryocoolers Part 1: Fundamentals”, Plenum Press, New York, 1983, by G. Walker, and the papers entitled “Cryocooler Applications”, Cold Facts, vol. 16, no. 1 (Winter 2000) by R. Radebaugh, pp. 1, 6, 7, 8, 16, 21, 24-25 and “Low-power Cryocooler Survey”, Cryogenics, vol. 42, (2002), by ter Brake and Wiegerinck, pp. 705-718.
One important property of a highly effective regenerator is that the regenerator material should have a large volumetric heat capacity. Most commercial regenerators today employ bronze or stainless steel screens or spheres to cool down to approximately 100 K, and lead (Pb) spheres to cool below 100 K, with 10 K being the no heat-load low temperature limit because the heat capacity of lead becomes extremely low at that temperature. Sometimes a combination of bronze or stainless steel and lead are used for cooling below 50 K with a layered regenerator bed for a single stage refrigerator. Or, a two stage refrigerator is used with a bronze alloy and stainless steel materials used in the high temperature stage and lead (Pb) used in the low temperature stage as a result of the heat capacity of lead not decreasing as quickly as that of the other materials below 100 K. Above 100 K, most metallic, non-magnetic materials have the same molar heat capacity, reaching the DuLong-Petit limit of 3R, where R (=8.314 J/mol K) is the universal gas constant. In general, the higher the heat capacity of the regenerator bed material, the greater the cooling power of a cryocooler, all other parameters being equal.
The potential use of lanthanide intermetallic compounds, which exhibit low magnetic ordering temperatures (e.g. less than 10 K), as cryogenic magnetic regenerator materials (refrigerant or cold accumulating materials) was pointed out nearly 25 years ago by Buschow et al. in an article entitled “Extremely Large Heat Capacities between 4 and 10 K, Cryogenics, vol. 15, (1975), pages 261-264. However, a practical lanthanide regenerator material was not developed and put into use until about 15 years later when the use of Er3Ni (a brittle intermetallic compound) as a low temperature stage regenerator material in a two-stage Gifford-McMahon cryocooler was proposed by Sahashi et al. in “New Magnetic Material R3T System with Extremely Large Heat Capacities Used as Heat Regenerators”, Adv. Cryogenic Eng., vol. 35, (1990), pages 1175-1182 and by Kuriyama et al. in “High Efficient Two-Stage GM Refrigerator with Magnetic Material in Liquid Helium Temperature Region”, Adv. Cryogenic Eng., vol. 35 (1990), pages 1261-1269.
These articles proposed the replacement of the lead (Pb) lower stage regenerator material with Er3Ni intermetallic compound material. Replacement of the lead lower stage regenerator material with Er3Ni material (a brittle intermetallic compound) permitted improved cooling to approximately 4.2 K instead of the approximately 10 K achievable with the previously used lead lower stage regenerator material with a reasonable refrigeration capacity at the lowest temperature. This improvement in cooling (i.e. to approximately 4.2 K) is attributable to the significantly higher heat capacity of Er3Ni than lead below 25 K (the heat capacity of lead becomes negligible below 10 K).
The Gschneidner and Pecharsky U.S. Pat. No. 5,537,826 issued Jul. 23, 1996, describes an improved regenerator for the low temperature stage (e.g. below 20 K) of a two stage Gifford-McMahon cryocooler. The patented regenerator comprises intermetallic compounds Er6Ni2Pb, Er6Ni2(SnxGa1−x), where x is greater than 0 and less than 1, and Er6Ni2Sn as a regenerator component.
An object of the present invention is to reduce the cost and to improve the reliability, efficiency and increase the cooling power of a cryocooler at low temperatures from about 2 K up to approximately 30 K.
Another object of the present invention is to utilize ductile magnetic rare earth (lanthanide) based intermetallic compounds, which can be easily fabricated into tough, non-brittle, corrosion resistant spherical powders, or thin sheets, or thin wires, or screens, or porous monolithic forms (such as cartridges), as the regenerator material.
Another object of the present invention is to provide a cryocooler with a regenerator having significantly higher heat capacity than the aforementioned previously used low temperature (less than 30K) regenerator materials and combinations thereof, such as Er3Ni, HoCu2 and PrxEr1−x.
More recently, HoCu2 (a brittle intermetallic compound) has replaced Er3Ni as the choice regenerator material for cooling down to approximately 2 K, see Satoh et al., “A Gifford-McMahon Cycle Cryocooler below 2 K”, Cryocoolers 11, R. G. Ross, Jr., editor, Kluwer Academic/Plenum Publishers, New York (2001), pp. 381-386. Also GdAlO3 (a brittle oxide has been suggested as a magnetic regenerator to reach temperatures below that attainable with either Er3Ni and HoCu2, i.e. about 2 K; it orders magnetically at 3.8 K. [Numazawa et al., “New Regenerator Material for Sub-4 K Cryocoolers”, Cryocoolers 11, R. G. Ross, Jr., editor, Kluwer Academic/Plenum Publishers, New York (2001), pp. 465-473].
The low temperature heat capacity properties of several rare earth—copper or silver binary compounds with the CsCl-type crystal, which have magnetic ordering temperatures below 20 K, have been reported in the literatue. However, none of the authors were aware of the ductile nature of these B2, CsCl-type compounds. These include: HoCu, ErCu, TmCu, PrAg, NdAg, (Pr1−xNdx)Ag, TbAg, ErAg, and TmAg. The first measurements were made on HoCu, ErCu, and TmCu, which were found to exhibit two or more magnetic ordering peaks: HoCu—at 13.4, 20 and 26.5 K; ErCu—at 10.9 and 13.8 K; and TmCu—at 6.7 and 7.7 K [“Competition Between Multi-qAntiferromagnetic Structures in Cubic Rare Earth-Copper Compounds”, J. Magn. Magn. Mater., vol. 21, (1980) by Morin and Schmidt, pp. 243-256]. The heat capacities of TbAg and ErAg from 0.5 and 21 K were measured and no magnetic transition was observed below 21 K for TbAg and three peaks at 11, 14.5, and 15.2 K for ErAg [“The Specific Heats of ErAg and TbAg Between 0.5 and 21 K”, J. Phys. F: Met. Phys., vol. 17, (1987) by R. W. Hill]. The heat capacity of ErAg is reasonably large at the 15 K double peak to warrant consideration as a regenerator material. Indeed Japanese scientists have proposed that ErAg be utilized as a regenerator material from 9 to 17 K. [“Evaluation of Low-temperature Specific Heats and Thermal Conductivities of Er—Ag Alloys as Regenerator Materials”, Jpn. J. Appl. Phys., vol. 35, (1996) by Biwa et al., pp. 2244-2248]. The heat capacities of PrAg, NdAg, and (Pr1−xNdx)Ag were measured from 2 to 25 K and only a single magnetic ordering peak was observed. The peak temperatures varied from 10 K for PrAg to 23 K for NdAg, while those for the ternary alloys were 11, 12.5, and 17 K for x=0.1 (also x=0.2), 0.5 (also x=0.6) and 0.8, respectively [“Studies of Low Temperature Specific Heats and Thermal Conductivities of CsCl-type (Pr1−xNdx)Ag (0≦x≦1) Intermetallic Compounds: Application to Regenerator Materials”, Jpn. J. Appl. Phys., vol. 36, (1997) by Yagi et al., pp. 5638-5643]. These authors found that the heat capacity maxima of the ternary alloys are generally significantly less than those of the two end members. They also suggested that PrAg would be a better regenerator alloy than Er3Ni at least over the 8 to 15 K temperature range. More recently, the large heat capacity of TmCu was confirmed, and that of TmAg was reported to be reasonably large at its magnetic ordering temperature, about 8 K [“The Similar Dependence of the Magnetocaloric Effect and Magneto-resistance in TmCu and TmAg Compounds and Its Implications”, J. Phys. Condens. Matter vol. 13, (2001) by Rawat and Das, pp. L379-L387]. This research substantiates the potential of TmCu as a low temperature cryocooler regenerator alloy and suggests that TmAg has only marginal utility as a regenerator material.
The present invention provides in one embodiment a cryocooler having improved cooling at the low temperature range or stages of operation, for example, 2 K up to 30 K, by using a passive magnetic regenerator comprising one or more regenerator components including a magnetic rare earth (lanthanide) metal as a component of a binary or ternary intermetallic compound and a non-rare earth metal as the other component. To reach temperatures of 30 K, standard cooling techniques are utilized, e.g. a Gifford-McMahon or a pulse tube cryocooler. The present invention envisions using one or more of the regenerator components in a particular embodiment to reach temperatures below 30 K, i.e. down to as low as about 2 K. An intermetallic compound is an ordered arrangement of the component atoms (two or more) on specific lattice sites in the crystal. The magnetic regenerator component(s) may comprise one or more rare earth (lanthanide) metals including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu with non-rare earth metals which form the CsCl, B2-type crystal structure (for example, Mg, Al, Co, Ni, Fe, Mn, Ga, Cu, Zn, Ru, Rh, Pd, Ag, Cd, In, Ir, Pt, Au, Hg, and Tl).
The rare earth (lanthanide) intermetallic compounds with the B2-type crystal structure can be used in the form of a layered regenerator bed comprising different metal and/or alloy layers in the form of wires, foils, jelly rolls, screens, monolithic porous cartridges, powders (spherical and non-spherical), or as a particulate bed comprising different metal particulate regions. The regenerator bed can include other materials such as HoCu2, Er3Ni, ErNi, PrxEr1−x, GdAlO3., lead, etc. to tailor regenerative properties of the regenerator bed. The magnetic regenerator is advantageous in that it can be tailored to improve cooling power and efficiency of the cryocooler in the temperature range or stage of operation from approximately 30 K to approximately 2 K.
Moreover, since the regenerator rare earth B2 intermetallic compounds are relatively ductile as compared, for example, to brittle intermetallic compounds (such as HoCu2, Er3Ni, ErNi), the regenerator layers or particulates will not attrite or comminute and pulverize in use of the regenerator. For example, the regenerator rare earth B2 intermetallic compounds typically have a ductility of at least about 5%, preferably about 10% and greater, tensile elongation prior to fracture when tensile tested in the as-cast or heat treated (annealed) condition at room temperature in ambient air pursuant to ASTM test E8-82 described in publication Annual Book of ASTM Standards published by American Society for Testing and Materials, 1985, V.301, West Conshohocken, Pa., incorporated herein by reference. Further, these rare earth intermetallic compounds can be readily fabricated into wires, screens, sheets, or spheres or porous monolithic form for use as regenerator components.
The advantage of the materials embodied in this invention, is that they can be easily and economically fabricated into a form which allows the design engineer to choose from spherical particles, wire screens, wire mesh, flat plates, jelly rolls, porous monolithic forms, etc. to construct the regenerator. Furthermore, since these materials are tough, they will not deform (as the soft lead spheres do) or comminute or decrepitate and pulverize (as the brittle intermetallic compounds do) under the cyclic high pressure gas flows used in present day cryocoolers. Furthermore, the embodied materials are oxidation resistant and do not become fine oxide powders when exposed to air as does Nd metal spheres or foil, which are used as regenerator materials in cryocoolers operating at 10 K or less.
The foregoing and other objects, features and advantages of the present invention will become apparent from the following more detailed description taken with the following drawings.
a and 4b are graphs which show the volumetric heat capacity of Er(Cu0.95Al0.05) and Er(Cu0.95Ga0.05), respectively, from 0 to 30 K compared to the low temperature prototype regenerator materials HoCu2, ErNi, Er50Pr50.
a and 5b are graphs which show the volumetric heat capacity of ErCu and Er(Cu0.95M0.05) doped alloys, where M═Mn and Fe (
a and 7b are graphs which show the volumetric heat capacity of ErCu and Er(Cu1−xNix) doped alloys, where x=0.05 and 0.10 (
a, 14b, and 14c are graphs which show the volumetric heat capacity of ErCu and (Er0.9R0.1)Cu doped alloys, where R═Ce, Pr, and Nd (
a and 33b are a plot of the magnetic ordering transition temperatures of the (Tm1−xErx)Cu alloys (
a, 39b, and 39c are schematic representations of a layered cryocooler regenerator for the lowest temperature stage of a sub 20 K cryocooler, where Tc is the lowest temperature and TH is the highest temperature; and A is the material with the lowest magnetic ordering temperature, B the material with the middle ordering temperature, and C the material with the highest magnetic ordering temperature for the 4 layered configuration (
Referring to
In general, the regenerator rare earth B2 intermetallic compounds described herein typically have a ductility of at least about 5%, preferably about 10% and greater, tensile elongation prior to fracture when tensile tested at room temperature in ambient air pursuant to ASTM test E8-82. These ductile rare earth intermetallic compounds can be readily fabricated into wires, screens, sheets, or spheres or porous monolithic form for use as regenerator components. It is for these reasons, a series of ErCu-base alloys were designed as improved cryocooler regenerator materials.
The shifting of magnetic ordering temperatures, in general, can be affected by alloying another element for either Er or for Cu, but there is no simple rule(s) to guide one in choosing the alloying agent or dopant element to achieve the desired properties, i.e. the appropriate magnetic ordering temperature with a reasonable volumetric heat capacity. As Gschneidner et al. [“Low Temperature Cryocooler Regenerator Materials”, Cryocooler 13, R. G. Ross, Jr., editor, Kluwer Academic/Plenum Publishers, New York, (2003) pages 457-465] point out, systematic trends are known but significant and unexpected deviations occur when the concentration of the dopant varies, and thus a blend of an Edisonian approach and systematics is required to find alloys with the desired properties. This observation of the unpredictability of magnetic ordering phenomena upon alloying was beautifully demonstrated in a later publication by Gschneidner et al. [“Effect of Interstitial Impurities on Magnetic Transitions of Er-rich PrxEr1−x Alloys”, J. Solid State Chem., vol. 171, (2003) by Gschneidner, et al., pp. 324-328] who found that several of the pure Er transitions (two second order and one first order magnetic transitions) disappear upon alloying, however, upon further Pr additions (>15 at. %) a new first order magnetic transition appears.
a and 5b are plots of the volumetric heat capacities of Cu substitute ErCu by 5% of Mn and Fe, and Co and Ni, respectively. The Mn and Fe substitution tends to shift the two peaks of pure ErCu closer together, but at the same time slightly reduce the volumetric heat capacity (
a and 7b summarize the effect of Ni substitutions for Cu in ErCu up to 20% on the ordering temperatures and the volumetric heat capacity. Ni additions tend to destroy the upper magnetic ordering peak; a 10% Ni addition is sufficient to do this (
The influence of Ag substitutions for Cu in ErCu is shown in
The volumetric heat capacity of (Er0.09R0.1)Cu doped alloys, where R═Sc, Y and Lu are presented in
The influence of the magnetic lanthanide metals for a 5% substitution of Er on the volumetric heat capacity of ErCu is shown in
The substitution of Al and Ga for Cu in TmCu results in the merging of the two peaks in pure TmCu into one peak with a substantial heat capacity, see
The effect of the substitution of Cu by 5% Co and Ni on the volumetric heat capacity of TmCu is shown in
The substitution of 2% ruthenium for Cu in TmCu is shown in
The substitution of the non-magnetic rare earth metals for Tm, i.e. (Tm0.95R0.05)Cu, is TmCu is shown in
The influence of Y additions (up to 15%) substituting for Tm in TmCu, i.e. (Tm1−xYx)Cu, are shown in
The substitution of 5% of the light magnetic lanthanide metals (Ce, Pr and Nd) for Tm in TmCu, i.e. (Tm0.95R0.05)Cu, wipes out the peaks of pure TmCu, see
The effect of the substitution of Er for Tm (Tm-rich alloys) and Tm for Er (Er-rich alloys) on the volumetric heat capacity in the (Tm1−xErx)Cu psuedobinary system is shown in
As shown in
Most of the ErM and TmM materials described above would be useful cryocooler regenerator materials for the low temperature stage of a multi-stage cryocooler to reach temperatures <15 K, and are excellent candidate materials to replace the prototype regenerator materials HoCu2 and ErNi. Not only are their volumetric heat capacities greater than (especially for HoCu2) or comparable to those of the two prototype materials, but they are ductile materials, which allows them to be fabricated into wires, sheets, screens, etc. in addition to spheres. The prototype materials are brittle and can only be fabricated into spheres to be utilized as regenerator materials. As it turns out, parallel plates (sheets) and screens configurations as regenerator components are much more efficient than spherical particles. Thus, the ErM and TmM materials have two distinct advantages over the HoCu2 and ErNi regenerator materials—the higher volumetric heat capacities and high ductilities.
The regenerator materials described hereabove were prepared as polycrystalline materials by arc-melting stoichiometric amounts of the component materials on a water cooled copper hearth under an argon atmosphere. The alloys generally were turned over six times although the ErIr alloy was turned over about 20 times and remelted to ensure a homogenous ingot. Weight losses after melting were negligible. The component metals used were purchased from various commercial sources. The rare earth metals were 95 to 98 atomic percent pure with the major impurities being O, C, and N while the non-rare earth metals were 99.9+ atomic percent pure. X-ray powder diffraction data were collected on an automated Scintag powder diffractometer using Cu Kalpha radiation to check on phase purity and crystallography of samples. Regenerator components of the invention may include one or more of H, O, C, N, and/or B as interstitial elements in an individual or collective amount up to about 5 atomic % of the compound depending upon the starting component materials for melting. All of the samples were found to be single-phase materials within the limitations of the diffraction technique (typically 2 to 5 volume % of an impurity phase). Most of the intermetallic compound samples were not heat treated because they were single phase alloys after arc-melting. ErRh and ErAu, however, were heat treated (annealed) for 335 hours (2 weeks) at 900° C. and rapidly quenched to room temperature to retain the B2 crystal structure. The heat capacities at constant pressure were measured using an adiabatic heat-pulse-type calorimeter from approximately 3.5 to approximately 350K in zero magnetic field. The calorimeter is described in U.S. Pat. No. 5,806,979 and by Pecharsky et al. in “A 3-350 K Fast Automatic Small Sample Calorimeter”, Rev. Sci. Instrum., vol. 68, pp. 4196-4207 (1997), which are incorporated herein by reference.
With the aid of
Tables 1 through 5 list materials that have reasonably high volumetric heat capacities over certain temperature ranges. That is: for temperatures below 4 K see Table 1; for temperatures between 4 and 10 K (replacement for HoCu2) see Table 2; for materials which have high heat capacities between HoCu2 and ErNi peaks (6 to 9 K) see Table 3; for temperatures between 8 and 13 K (replacements for ErNi) see Table 4; and for temperatures between 11K and 17K, see Table 5.
Examples of some regenerator configurations for the low temperature stage for a high performance cryocooler to reach temperatures below 15 K are described below.
With
For a cryocooler to reach a temperature in the 4 to 9 K range using the layering sequence shown in
Another four layer sequence (
An alternate 3 layer configuration to reach a temperature in the 4 to 9 K range is shown in
The utilization of a material given in Table 3 in section or layer A plus a compound presented in Table 4 as the section or layer B component in the three layer configuration (
A four layer cryocooler regenerator (
Another four layer regenerator configuration (
An alternative solution to reach the 8 to 12 K temperature range is the utilization of a three layered regenerator configuration (
Another three layered arrangement (
For a cryocooler to reach an ultimate low temperature of 12 to 16 K a three layered regenerator (
The two layered regenerator configuration as shown in
An alternate material chosen from Table 4 as the section A component in a two layered regenerator (
The layers described above in the Examples may comprise different layers of spherical powder or other particles, or other forms of the materials.
The regenerator intermetallic compound may also include other metals or non-metals preferably selected from Li, B, C, Si, P, Ga, Ge, Mn, Fe and other metals or non-metals to modify a particular property of the regenerator component such that the compound retains the B2 (CsCl-type) ordered crystal structure as apparent below.
Tables 1, 2, 3, 4, and 5 illustrate intermetallic compounds of general types represented by RM, Er(Mx,M′1−x) and Tm(Mx,M′1−x) where M and M′ represent one or more non-rare earth metals; (Er1−x,Rx)M and (Tm1−x,Rx)M where R represents one or more rare earth metals and M represents one or more non-rare earth metals; and Er(Mx,M′1−x) and Tm(Mx,M′1−x) where M and M′ represent one or more non-rare earth metals.
This application claims benefits and priority of provisional application Ser. No. 60/546,740 filed Feb. 23, 2004.
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Number | Date | Country | |
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20050217280 A1 | Oct 2005 | US |
Number | Date | Country | |
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60546740 | Feb 2004 | US |